Aluminum Anodizing Design Guide

There are some important factors to consider in Aluminum Anodizing to produce better quality finish. The following guide is aimed at giving useful insights for anodizing design.

The things to avoid when designing for an anodized finish:

  • blind holes
  • hollow weldments
  • steel inserts
  • sharp corners
  • heavy to thin cross sections

Chemical conversion coatings

Conversion coatings are used to add protective surface layer on a metal or substrate intended to prevent corrosion, enhance the adhesion of paint finish or other coatings, and for improved electrical application. Two classes are involved in chemical conversions: (1) Class 1A, chemical conversion coatings are to provide corrosion resistance when left unpainted; it also helps improve adhesion property of finish systems. (2) Class 3, wherein the chemical conversion coatings are to be used as a corrosion preventative film.

Weldments and Welded aluminum parts

When two or more aluminum parts are welded together, it causes acid to be confined in the weld and the area around it. There are also color variations present particularly when a welding rod alloy is vastly different from the alloy used to make the part. Aside from that, halos also appear around the welds caused by high temperature that is applied in the welding process.

Hardcoat and Thread Coating

Hardcoat thickness is typically 0.002" (0.0508 mm). Half the coating thickness is build-up and half is penetration into the base metal. For the threaded rod below, the diameter increased by 0.002" since half of the coating thickness (0.001") built up the diameter on each side of the rod.

hardcoat and thread coating

Thread Coating - GMP Plating, Inc.

Table of Hardcoat Properties


65 - 70 Rockwell C, 850 to 900 DPH, harder than hard chrome plate


Dark Gray to Black

Coating Thickness

0.002" average, 0.015" for salvage purposes on selected alloys


Hardcoat is non-conductive and can withstand 800 volts per 0.001" thickness.


Hardcoat may be dyed most colors but tends to come out dark.


Hardcoat can be ground, lapped, honed or polished.


between 106to 1012Ohm-cm


Hardcoat may be Dichromate, Ni-Acetate, hot water, or Teflon sealed.

Coating Thickness vs. Corner Radius

Nominal Coating Thickness (inches)

Radius on edge & inside corner


~ 1/32 inch


~ 1/16 inch


~ 3/32 inch


~ 1/8 inch

Hardcoat Blind Hole vs Through Holes

Through holes (TH on illustration below) will hardcoat evenly up to twice the length of the di diameter (X)

Blind holes (BH on drawing below) will only hardcoat to a depth equal to the diameter of the hole (X=Y)

Coating Thickness, Color and Alloys

Coating Thickness, Color and Alloys

See chart at below. Example: Alloy 2024 may have a maximum coating thickness of 0.005" and will be gray in color.

++Note: Coatings over 0.0035" tend to chip and become milky in color and should only be used in the salvage of parts.

Maximum Coating Thickness in Mil Diagram


Masking is necessary where no build up is desired or when a part requires both anodizing & chemical conversion. All threaded holes, 0.25" or smaller, are usually masked when hardcoating unless otherwise specified. Holes with heli-coils must be masked. Any dissimilar metal (steel, brass, bronze) or any form of plating substrate will burn off in the anodizing tank unless it is masked.

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